U.S. patent application number 14/126393 was filed with the patent office on 2014-05-08 for method for transmitting and receiving data unit based on uplink multiple user multiple input multiple output transmission and apparatus for the same.
This patent application is currently assigned to LG ELECTRONICS INC.. The applicant listed for this patent is Hyang Sun You. Invention is credited to Hyang Sun You.
Application Number | 20140126509 14/126393 |
Document ID | / |
Family ID | 47357289 |
Filed Date | 2014-05-08 |
United States Patent
Application |
20140126509 |
Kind Code |
A1 |
You; Hyang Sun |
May 8, 2014 |
METHOD FOR TRANSMITTING AND RECEIVING DATA UNIT BASED ON UPLINK
MULTIPLE USER MULTIPLE INPUT MULTIPLE OUTPUT TRANSMISSION AND
APPARATUS FOR THE SAME
Abstract
In an aspect, a method of receiving a data unit, performed by an
Access Point (AP), in a Wireless LAN (WLAN) system is provided. The
method includes transmitting a signal protection frame, the signal
protection frame comprising a group ID field indicating a Multiple
Input Multiple Output (MIMO) transmission STA group including a
first station (STA) and a second STA; and, a spatial stream field
indicating a number of spatial streams allocated to each of member
STAs included in the MIMO transmission STA group; receiving a first
preamble for a first data unit from the first STA; receiving a
second preamble for a second data unit from the second STA; and
simultaneously receiving the first data unit from the first STA and
the second data unit from the second STA.
Inventors: |
You; Hyang Sun; (Anyang-si,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
You; Hyang Sun |
Anyang-si |
|
KR |
|
|
Assignee: |
LG ELECTRONICS INC.
Seoul
KR
|
Family ID: |
47357289 |
Appl. No.: |
14/126393 |
Filed: |
February 23, 2012 |
PCT Filed: |
February 23, 2012 |
PCT NO: |
PCT/KR2012/001372 |
371 Date: |
December 13, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61497062 |
Jun 15, 2011 |
|
|
|
Current U.S.
Class: |
370/329 |
Current CPC
Class: |
H04B 7/0697 20130101;
H04L 27/2602 20130101; H04B 7/04 20130101; H04B 7/0413 20130101;
H04L 5/0053 20130101; H04B 7/0452 20130101; H04W 84/12 20130101;
H04W 72/046 20130101 |
Class at
Publication: |
370/329 |
International
Class: |
H04L 5/00 20060101
H04L005/00; H04W 84/12 20060101 H04W084/12; H04B 7/04 20060101
H04B007/04 |
Claims
1. A method of receiving a data unit in a Wireless LAN (WLAN)
system, comprising: transmitting, by an Access Point (AP), a signal
protection frame, the signal protection frame comprising a group ID
field indicating a Multiple Input Multiple Output (MIMO)
transmission STA group including a first station (STA) and a second
STA; and, a spatial stream field indicating a number of spatial
streams allocated to each of member STAs included in the MIMO
transmission STA group; receiving, by the AP, a first preamble for
a first data unit from the first STA; receiving, by the AP, a
second preamble for a second data unit from the second STA; and
simultaneously receiving, by the AP, the first data unit from the
first STA and the second data unit from the second STA.
2. The method of claim 1, wherein the first preamble comprises: a
first Long Training Field (LTF) for estimating a first MIMO channel
between the AP and the first STA; and a first signal field
comprising control information for interpreting the first data
unit.
3. The method of claim 2, wherein the second preamble comprises: a
second LTF for estimating a second MIMO channel between the AP and
the second STA; and, a second signal field comprising control
information for interpreting the second data unit.
4. The method of claim 3, wherein the second preamble is received
after the first preamble has been received.
5. The method of claim 3, wherein a time interval where the first
preamble is received overlaps with a time interval where the second
preamble is received.
6. The method of claim 5, wherein a sequence constituting the first
LTF and a sequence constituting the second LTF are orthogonal to
each other.
7. The method of claim 6, wherein time when the first preamble
starts to be received and time when the second preamble starts to
be received are identical with each other.
8. The method of claim 7, further comprising receiving, by the AP,
dummy bits from the second STA between when a transmission of the
second LTF is finished and when a reception of the second data unit
is started, if a length of the first LTF is longer than a length of
the second LTF.
9. The method of claim 1, further comprising transmitting, by the
AP, information indicating multi-user transmission, before the
transmitting the signal protection frame.
10. The method of claim 9, wherein the information indicating the
multi-user transmission is included in a Clear To Send (CTS) frame
transmitted by the AP in response to a Request To Send (RTS) frame
for uplink transmission, transmitted from the first STA to the
AP.
11. The method of claim 1, further comprising receiving, by the AP,
Multi-User (MU) indication information indicating that data are to
be transmitted through MU transmission, from the first STA.
12. The method of claim 11, wherein the MU indication information
is transmitted in an RTS frame by the first STA for uplink
transmission.
13. A wireless apparatus, comprising: a transceiver configured to
transmit and receive a radio signal; and a processor functionally
coupled to the transceiver, wherein the processor is configured
for: transmitting, a signal protection frame, the signal protection
frame comprising a group ID field indicating a Multiple Input
Multiple Output (MIMO) transmission STA group including a first
station (STA) and a second STA; and, a spatial stream field
indicating a number of spatial streams allocated to each of member
STAs included in the MIMO transmission STA group; receiving a first
preamble for a first data unit from the first STA; receiving a
second preamble for a second data unit from the second STA; and
simultaneously receiving the first data unit from the first STA and
the second data unit from the second STA.
Description
TECHNICAL FIELD
[0001] The present invention relates to a Wireless LAN (WLAN)
system and, more particularly, to a method of transmitting and
receiving a data unit based on an uplink Multiple User-Multiple
Input Multiple Output (MU-MIMO) transmission scheme in a WLAN
system and an apparatus for supporting the method.
BACKGROUND ART
[0002] With the advancement of information communication
technologies, various wireless communication technologies have
recently been developed. Among the wireless communication
technologies, a wireless local area network (WLAN) is a technology
whereby Internet access is possible in a wireless fashion in homes
or businesses or in a region providing a specific service by using
a portable terminal such as a personal digital assistant (PDA), a
laptop computer, a portable multimedia player (PMP), etc.
[0003] The initial WLAN technology was able to support the rate of
1 to 2 Mbps through frequency hopping, band spreading, and infrared
communication by using a 2.4 GHz frequency in accordance with the
IEEE 802.11 standard, but the recent WLAN technology can support a
maximum rate of 54 Mbps by using Orthogonal Frequency Division
Multiplexing (OFDM). Furthermore, in IEEE 802.11, the standards of
various technologies, such as the improvements of Quality of
Service (QoS), the compatibility of Access Point (AP) protocols,
security enhancement, radio resource measurement, a wireless access
vehicular environment, fast roaming, a mesh network, interworking
with an external network, and wireless network management, are put
to practical use or being developed.
[0004] Furthermore, in order to overcome limitations to the
communication speed that has been considered to be weakness in the
WLAN, an IEEE 802.11n standard has recently been regulated as a
technology standard. An object of the IEEE 802.11n standard is to
increase the speed and reliability of a network and to expand the
coverage of a wireless network. More particularly, in order to
support a High Throughput (HT) having a maximum data processing
speed of 540 Mbps or higher, minimize a transmission error, and
optimize the data rate, the IEEE 802.11n standard is based on
Multiple Inputs and Multiple Outputs (MIMO) technology in which
multiple antennas are used on both sides of a transmitter and a
receiver. Furthermore, the standard may use not only a coding
scheme for transmitting several redundant copies in order to
increase data reliability, but also Orthogonal Frequency Division
multiplexing (OFDM) in order to increase the speed.
[0005] In a High Throughput (HT) WLAN system based on IEEE 802.11n,
a diversity gain and a gain related to an increase of the channel
capacity could be obtained by using a Single User (SU) MIMO
transmission scheme between an Access Point (AP) and a station
(STA). In the SU-MIMO transmission scheme, the degree of freedom of
space may be expanded by increasing the number of antennas for
transmitting and receiving a radio signal, thereby contributing to
the improved performance of a physical layer.
[0006] The HT WLAN system has introduced an HT green field PPDU
format which may be used in a system including only HT STAs, in
addition to the Physical Layer Convergence Procedure (PLCP)
Protocol Data Unit (PPDU) format which supports a legacy STA
operated according to the standards of the existing WLAN system.
Furthermore, the HT WLAN system supports an HT mixed PPDU format
which is a PPDU format designed to support an HT system in a system
where a legacy STA and an HT STA coexist.
[0007] As the spread of the WLAN is activated and applications
using the WLAN are diversified, there is a need for a new WLAN
system for supporting the throughput higher than the data
processing speed supported by the IEEE 802.11n standard. The
next-generation WLAN system supporting a Very High Throughput (VHT)
is the next version of the IEEE 802.11n WLAN system and is one of
IEEE 802.11 WLAN systems which are recently newly proposed in order
to support the data processing speed of 1 Gbps or higher in an MAC
Service Access Point (SAP).
[0008] The next-generation WLAN system allows a plurality of STAs
to access and use channels at the same time in order to efficiently
use radio channels. To this end, the next-generation WLAN system
supports the transmission of a downlink MU-MIMO scheme using
multiple antennas. It is here assumed that the downlink is a link
along which data is transmitted from an AP to an STA. The AP may
perform Spatial Division Multiple Access (SDMA) transmission in
which spatially multiplexed data is transmitted to a plurality of
STAs at the same time. The overall throughput of the WLAN system
can be increased by distributing a plurality of spatial streams
into a plurality of STAs using a plurality of antennas and
transmitting data to the STAs at the same time.
[0009] Meanwhile, if only downlink MU-MIMO transmission is
supported, when STAs transmit frames to an AP, a medium access
period is divided for every STA, and the STAs independently transit
the frames to the AP. In this case, the improvement of the
throughput cannot be expected, as compared with the existing WLAN
system, because a common frame transmission scheme or an SU-MIMO
transmission scheme is used in the uplink transmission period from
the STA. On the other hand, if STAs can transmit frames for traffic
processing to an AP at the same time, the overall throughput of the
WLAN system may be further improved. In order to further improve
the efficiency of a WLAN system, there is a need for a method of
transmitting and receiving a data unit which supports uplink
MU-MIMO transmission.
SUMMARY OF INVENTION
Technical Problem
[0010] It is an object of the present invention to provide a method
of transmitting and receiving a data unit based on uplink MU-MIMO
in a WLAN system and an apparatus for supporting the method.
Solution to Problem
[0011] In an aspect, a method of receiving a data unit, performed
by an Access Point (AP), in a Wireless LAN (WLAN) system is
provided. The method includes transmitting a signal protection
frame, the signal protection frame comprising a group ID field
indicating a Multiple Input Multiple Output (MIMO) transmission STA
group including a first station (STA) and a second STA; and, a
spatial stream field indicating a number of spatial streams
allocated to each of member STAs included in the MIMO transmission
STA group; receiving a first preamble for a first data unit from
the first STA; receiving a second preamble for a second data unit
from the second STA; and simultaneously receiving the first data
unit from the first STA and the second data unit from the second
STA.
[0012] The first preamble may include a first Long Training Field
(LTF) for estimating a first MIMO channel between the AP and the
first STA and a first signal field comprising control information
for interpreting the first data unit.
[0013] The second preamble may include a second LTF for estimating
a second MIMO channel between the AP and the second STA and a
second signal field comprising control information for interpreting
the second data unit.
[0014] The second preamble may be received after the first preamble
has been received.
[0015] A time interval where the first preamble is received may
overlap with a time interval where the second preamble is
received.
[0016] A sequence constituting the first LTF and a sequence
constituting the second LTF may be orthogonal to each other.
[0017] Time when the first preamble starts to be received and time
when the second preamble starts to be received may be identical
with each other.
[0018] The method may further include receiving dummy bits from the
second STA between when a transmission of the second LTF is
finished and when a reception of the second data unit is started,
if a length of the first LTF is longer than a length of the second
LTF.
[0019] The method may further include transmitting information
indicating multi-user transmission, before the transmitting the
signal protection frame.
[0020] The information indicating the multi-user transmission may
be included in a Clear To Send (CTS) frame transmitted by the AP in
response to a Request To Send (RTS) frame for uplink transmission,
transmitted from the first STA to the AP.
[0021] The method may further include receiving Multi-User (MU)
indication information indicating that data are to be transmitted
through MU transmission, from the first STA.
[0022] The MU indication information may be transmitted in an RTS
frame by the first STA for uplink transmission.
[0023] In another aspect, a wireless apparatus is provided. The
apparatus includes a transceiver configured to transmit and receive
a radio signal; and a processor functionally coupled to the
transceiver. The processor is configured for: transmitting, a
signal protection frame, the signal protection frame comprising a
group ID field indicating a Multiple Input Multiple Output (MIMO)
transmission STA group including a first station (STA) and a second
STA; and, a spatial stream field indicating a number of spatial
streams allocated to each of member STAs included in the MIMO
transmission STA group; receiving a first preamble for a first data
unit from the first STA; receiving a second preamble for a second
data unit from the second STA; and simultaneously receiving the
first data unit from the first STA and the second data unit from
the second STA.
Advantageous Effects of Invention
[0024] An Access Point (AP) provides stations (STAs) associated
with the AP with a training sequence and control information for
uplink Multiple User-Multiple Input Multiple Output (MU-MIMO)
transmission. STAs transmit respective preambles for interpreting
data units and, at the same time, transmit the data units to an
AP.
[0025] Each STA may process uplink traffic within the same time
interval. Through such single user transmission, the overall
throughput of a WLAN system can be improved as compared with the
existing WLAN system in which time is divided into STAs and uplink
traffic is processed.
BRIEF DESCRIPTION OF DRAWINGS
[0026] FIG. 1 shows an IEEE 802.11 physical layer (PHY)
architecture.
[0027] FIG. 2 is a block diagram showing an example of a PPDU
format according to an embodiment of the present invention;
[0028] FIG. 3 is a flowchart illustrating a method of transmitting
a PPDU using an MU-MIMO transmission scheme according to an
embodiment of the present invention;
[0029] FIG. 4 is a diagram showing a change of a scrambling
sequence for a Clear To Send (CTS) frame according to an embodiment
of the present invention;
[0030] FIG. 5 is a block diagram showing a VHT-SIG protection frame
according to an embodiment of the present invention;
[0031] FIG. 6 is a diagram showing a traffic queue for each of
STAs;
[0032] FIG. 7 is a flowchart illustrating an example of a method of
transmitting a PPDU according to an embodiment of the present
invention;
[0033] FIG. 8 is a flowchart illustrating an example of a method of
transmitting a PPDU based on an SDMA scheme according to an
embodiment of the present invention;
[0034] FIG. 9 is a flowchart illustrating another example of a
method of transmitting a PPDU based on an SDMA scheme according to
an embodiment of the present invention;
[0035] FIG. 10 is a flowchart illustrating yet another example of a
method of transmitting a PPDU based on an SDMA scheme according to
an embodiment of the present invention; and
[0036] FIG. 11 is a block diagram showing a wireless apparatus in
which the embodiments of the present invention may be
implemented.
MODE FOR THE INVENTION
[0037] A Wireless Local Area Network (WLAN) system to which
embodiments of the present invention may be applied includes one or
more Basic Service Sets (BSSs). The BSS is a set of stations (STAs)
which can communicate with each other through successful
synchronization, and the BSS is not a concept indicating a specific
area.
[0038] An infrastructure basic service set (BSS) includes one or
more non-AP STAs STA1, STA2, STA3, STA4, and STA5, an AP (Access
Point) providing distribution service, and a Distribution System
(DS) connecting a plurality of APs. In the infrastructure BSS, an
AP manages the non-AP STAs of the BSS.
[0039] On the other hand, an Independent BSS (IBSS) is operated in
an Ad-Hoc mode. The IBSS does not have a centralized management
entity for performing a management function because it does not
include an AP. That is, in the IBSS, non-AP STAs are managed in a
distributed manner. In the IBSS, all STAs may be composed of mobile
STAs. All the STAs form a self-contained network because they are
not allowed to access the DS.
[0040] An STA is a certain functional medium, including Medium
Access Control (MAC) and wireless-medium physical layer interface
satisfying the Institute of Electrical and Electronics Engineers
(IEEE) 802.11 standard. Hereinafter, the STA refers to both an AP
and a non-AP STA.
[0041] A non-AP STA is an STA which is not an AP. The non-AP STA
may also be referred to as a mobile terminal, a wireless device, a
wireless transmit/receive unit (WTRU), a user equipment (UE), a
mobile station (MS), a mobile subscriber unit, or simply a user.
For convenience of explanation, the non-AP STA will be hereinafter
referred to the STA.
[0042] The AP is a functional entity for providing connection to
the DS through a wireless medium for an STA associated with the AP.
Although communication between STAs in an infrastructure BSS
including the AP is performed via the AP in principle, the STAs can
perform direct communication when a direct link is set up. The AP
may also be referred to as a central controller, a base station
(BS), a node-B, a base transceiver system (BTS), a site controller,
etc.
[0043] A plurality of infrastructure BSSs including the BSS can be
interconnected by the use of the DS. An extended service set (ESS)
is a plurality of BSSs connected by the use of the DS. APs and/or
STAs included in the ESS can communicate with each another. In the
same ESS, an STA can move from one BSS to another BSS while
performing seamless communication.
[0044] In a WLAN system based on IEEE 802.11, a basic access
mechanism of a medium access control (MAC) is a carrier sense
multiple access with collision avoidance (CSMA/CA) mechanism. The
CSMA/CA mechanism is also referred to as a distributed coordinate
function (DCF) of the IEEE 802.11 MAC, and basically employs a
"listen before talk" access mechanism. In this type of access
mechanism, an AP and/or an STA senses a wireless channel or medium
before starting transmission. As a result of sensing, if it is
determined that the medium is in an idle status, frame transmission
starts by using the medium. Otherwise, if it is sensed that the
medium is in an occupied status, the AP and/or the STA does not
start its transmission but sets and waits for a delay duration for
medium access.
[0045] The CSMA/CA mechanism also includes virtual carrier sensing
in addition to physical carrier sensing in which the AP and/or the
STA directly senses the medium. The virtual carrier sensing is
designed to compensate for a problem that can occur in medium
access such as a hidden node problem. For the virtual carrier
sending, the MAC of the WLAN system uses a network allocation
vector (NAV). The NAV is a value transmitted by an AP and/or an
STA, currently using the medium or having a right to use the
medium, to anther AP or another STA to indicate a remaining time
before the medium returns to an available state. Therefore, a value
set to the NAV corresponds to a period reserved for the use of the
medium by an AP and/or an STA transmitting a corresponding
frame.
[0046] The AP and/or the STA may perform a procedure of exchanging
a request to send (RTS) frame and a clear to send (CTS) frame to
announce that it intends to access a medium. The RTS frame and the
CTS frame include information indicating a time duration reserved
for access of a radio medium required to transmit and receive an
acknowledgement (ACK) frame when an actual data frame transmission
and reception ACK is supported. Upon receiving an RTS frame
transmitted from an AP and/or an STA intending to transmit a frame
or upon receiving a CTS frame transmitted from a frame transmission
target STA, another STA can be configured not to access to the
medium for the time duration indicated by the information included
in the RTS/CTS frame. This can be implemented by configuring an NAV
for the time duration.
[0047] Meanwhile, if channel sensing is always performed for frame
transmission and reception, it causes persistent power consumption
of the STA. Since power consumption in a reception state is not
much different from power consumption in a transmission state, if
the reception state needs to be continuously maintained, relatively
great power consumption is generated in an STA that operates by
using a battery. Therefore, when the STA senses a channel by
persistently maintaining a reception standby state in a WLAN
system, ineffective power consumption may be caused without a
special synergy effect in terms of a WLAN throughput, and thus it
may be inappropriate in terms of power management.
[0048] To compensate for the problem above, the WLAN system
supports a power management (PM) mode of the STA. A power
management (PM) mode of a STA is classified into an active mode and
a power save (PS) mode in a WLAN system. Basically, the STA
operates in the active mode. When operating in the active mode, the
STA can operate in an awake state so that a frame can be received
all the time.
[0049] When operating in the PS mode, the STA operates by
transitioning between a doze state and the awake state. When
operating in the doze state, the STA operates with minimum power,
and does not receive a radio signal, including a data frame,
transmitted from an AP. In addition, the STA operating in the doze
state does not perform channel sensing.
[0050] The longer the STA operates in a doze state, the less the
power consumption is, and thus the longer the STA operates.
However, since a frame cannot be transmitted and received in the
doze state, the STA cannot operate long unconditionally. If the STA
operating in the doze state has a frame to be transmitted to the
AP, the STA can transition to an awake state to transmit the frame.
However, if the AP has a frame to be transmitted to the STA
operating in the doze state, the STA cannot receive the frame and
cannot know that there is the frame to be received. Therefore, the
STA may need to know whether there is the frame to be transmitted
to the STA, and if the frame exists, may require an operation for
transitioning to the awake state in accordance with a specific
period.
[0051] FIG. 1 shows an IEEE 802.11 physical layer (PHY)
architecture.
[0052] The IEEE 802.11 PHY architecture includes a PHY layer
management entity (PLME), a physical layer convergence procedure
(PLCP) sub-layer 110, and a physical medium dependent (PMD)
sub-layer 100. The PLME provides a PHY management function in
cooperation with a MAC layer management entity (MLME). The PLCP
sub-layer 110 located between a MAC sub-layer 120 and the PMD
sub-layer 100 delivers to the PMD sub-layer 100 a MAC protocol data
unit (MPDU) received from the MAC sub-layer 120 under the
instruction of the MAC layer, or delivers to the MAC sub-layer 120
a frame received from the PMD sub-layer 100. The PMD sub-layer 100
is a lower layer of the PDCP sub-layer and serves to enable
transmission and reception of a PHY entity between two STAs through
a radio medium. The MPDU delivered by the MAC sub-layer 120 is
referred to as a physical service data unit (PSDU) in the PLCP
sub-layer 110. Although the MPDU is similar to the PSDU, when an
aggregated MPDU (A-MPDU) in which a plurality of MPDUs are
aggregated is delivered, individual MPDUs and PSDUs may be
different from each other.
[0053] The PLCP sub-layer 110 attaches an additional field
including information required by a PHY transceiver to the MPDU in
a process of receiving the MPDU from the MAC sub-layer 120 and
delivering a PSDU to the PMD sub-layer 100. The additional field
attached in this case may be a PLCP preamble, a PLCP header, tail
bits required on a data field, etc. The PLCP preamble serves to
allow a receiver to prepare a synchronization function and antenna
diversity before the PSDU is transmitted. The PLCP header includes
a field that contains information on a PLCP protocol data unit
(PPDU) to be transmitted, which will be described below in greater
detail with reference to FIG. 2.
[0054] The PLCP sub-layer 110 generates a PLCP protocol data unit
(PPDU) by attaching the aforementioned field to the PSDU and
transmits the generated PPDU to a reception STA via the PMD
sub-layer. The reception STA receives the PPDU, acquires
information required for data recovery from the PLCP preamble and
the PLCP header, and recovers the data.
[0055] The next-generation WLAN system supports a downlink MU-MIMO
transmission scheme in which an AP transmits data to a plurality of
STAs. Accordingly, an AP and STAs may have a plurality of antennas.
For the Very High Throughput (VHT) to be supported by the
next-generation WLAN system, a physical layer supports MU-MIMO and
Orthogonal Frequency Division multiplexing (OFDM). For this,
channel bandwidths of 20 MHz, 40 MHz, 80 MHz, contiguous 160 MHz,
and non-contiguous 160 MHz (80+80 MHz) are supported. Binary Phase
Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK),
Quadrature Amplitude Modulation (16QAM), 64QAM, and 256 QAM are
applied to each subcarrier. Furthermore, the coding rates of 1/2,
2/3, 3/4, and are supported through convolutional coding or Forward
Error Correction (FEC) using Low Density Parity Check (LDPC) codes
or both as coding schemes.
[0056] Meanwhile, according to the downlink MU-MIMO transmission
scheme provided by the next-generation WLAN system, STAs cannot
cooperate with each other because one AP performs transmission and
a plurality of STAs performs reception at the same time.
Accordingly, the AP must know information about a channel between
the AP and the plurality of STAs, and the AP precodes a data
sequence based on the channel information and transmits the data
sequence.
[0057] In general, precoding includes linear precoding and
nonlinear precoding. A representative example of the linear
precoding is a channel inversion (zero forcing) scheme for removing
interference between users. The channel inversion scheme is
disadvantageous in that a noise enhancement phenomenon is
generated. In order to reduce this problem, a regularized channel
inversion (minimum mean squared error) scheme is used. The linear
precoding scheme has lower complexity than the nonlinear precoding
scheme, but has relatively lower performance than the nonlinear
precoding scheme.
[0058] The nonlinear precoding scheme includes a vector
perturbation scheme for perturbing transmission data in order to
reduce the noise enhancement problem and a Dirty Paper Coding (DPC)
scheme for obtaining the entire channel capacity in theory. The
nonlinear precoding scheme has relatively higher complexity than
the linear precoding scheme, but has better performance than the
linear precoding scheme.
[0059] In a WLAN system supporting an MIMO transmission scheme, the
accuracy of channel information owned by an AP has a great
influence on WLAN performance. A method of an STA informing an AP
of channel information includes a channel sounding method (implicit
feedback method) in which the STA transmits a predetermined pattern
to the AP so that the AP can estimate the channel information and
an explicit feedback method in which the STA inform the AP of the
channel information in the form of data.
[0060] In a WLAN system supporting MIMO transmission, if channel
information is fed back in the form of a data type, there may be a
problem in that radio resources for uplink communication are
occupied because the size of the feedback channel information is
increased. In general, since an AP does not know all pieces of
channel information, a scheme of each STA previously determining a
preferred beam and informing the AP of beamforming matrix
information or a scheme of using a predetermined beam or a codebook
between the AP and STAs may be used instead of a precoding scheme
using channel information without change.
[0061] As an example of the scheme using a predetermined beam, a
method of each STA selecting a beam having a maximum Signal to
Interference plus Noise Ratio (SINR), from among beams, and
transmitting the selected beam with an index of the beam to an AP
may be used. This scheme is advantageous in that feedback
information is relatively simple because the AP allocates the most
preferred STAs to respective beams and transmitting the beams at
the same time. This scheme, however, is problematic in that
performance is good when the number of STAs is many, but
performance may be abruptly deteriorated when the number of STAs is
small. As another illustrative scheme, a method of an STA providing
preferred beamforming matrix information to an AP and the AP
performing precoding using the beamforming matrix information is
advantageous in that deterioration of performance is not great as
compared with the channel sounding method and calibration is not
required.
[0062] An AP generates a PPDU based on the obtained channel
information and transmits the channel information to a plurality of
STAs using the MU-MIMO transmission scheme. The channel information
is applied to the training field of the PPDU.
[0063] FIG. 2 is a block diagram showing an example of a PPDU
format according to an embodiment of the present invention.
[0064] Referring to FIG. 2, a PPDU 200 may include a Legacy Short
Training Field (L-STF) 210, a Legacy Long Training Field (L-LTF)
220, a Legacy Signal (L-SIG) field 230, a VHT-SIG A field 240, a
VHT-STF 250, a VHT-LTF 260, a VHT-SIG B field 270, and a data field
280.
[0065] A PLCP sublayer constituting a PHY layer adds information
necessary for a PHY Service Data Unit (PSDU), received from a
Medium Access Control (MAC) layer, to the PSDU, converts the PSDU
into the data field 280, generates the PPDU 200 by adding fields,
such as the L-STF 210, the L-LTF 220, the L-SIG field 230, the
VHT-SIG A field 240, the VHT-STF 250, the VHT-LTF 260, and the
VHT-SIG B field 270, to the data field 280, and transmits the PPDU
200 to one or more STAs through a Physical Medium Dependent (PMD)
sublayer forming the PHY layer.
[0066] The L-STF 210 is used for frame timing acquisition,
Automatic Gain Control (AGC) convergence, coarse frequency
acquisition, etc.
[0067] The L-LTF 220 is used to estimate a channel for demodulating
the L-SIG field 230 and the VHT-SIG A field 240.
[0068] The L-SIG field 230 is used for an STA to receive a PPDU and
obtain data by interpreting the PPDU. In particular, an L-STA not
supporting the VHT may obtain data based on information included in
the L-SIG field.
[0069] The L-SIG field 230 includes a rate subfield, a length
subfield, a parity bit, and a tail field. The rate subfield is set
to a value indicating the bit rate for data to be now
transmitted.
[0070] The length subfield is set to a value indicating the octet
length of a PSDU that an MAC layer requests a PHY layer to transmit
the PSDU. Here, an L-LENGTH parameter related to information about
the octet length of the PSDU is determined based on a TXTIME
parameter related to the transmission time. TXTIME indicates the
transmission time determined by the PHY layer in order to transmit
the PPDU, including the PSDU, according to the transmission time
requested by the MAC layer in order to transmit the Physical
Service Data Unit (PSDU). Accordingly, the L-LENGTH parameter is
related to time, and thus the length subfield included in the L-SIG
field 230 includes information related to the transmission
time.
[0071] The VHT-SIG A field 240 is related to common control
information necessary for STAs paired with an AP, and it includes
control information for interpreting the received PPDU 200. The
VHT-SIG A field 240 may be divided into a VHT-SIG A1 field and a
VHT-SIG B field. The VHT-SIG A1 field includes information about a
used channel bandwidth, ID information about whether Space Time
Block Coding (STBC) is used, a group identifier (i.e., ID
information about a target transmission STA group), and information
about spatial streams allocated to an STA included in a target
transmission group STA indicated by the group identifier. The
VHT-SIG A2 field may include information related to a short Guard
Interval (GI) of a target transmission STA, a Modulation and Coding
Scheme (MCS) for a single user, a channel coding type for
multi-user, information about beamforming, information about
redundancy bits for Cyclic Redundancy Checking (CRC), and
information about the tail bits of a convolution decoder. The
VHT-SIG A1 field and the VHT-SIG A2 field may be transmitted
through respective OFDM symbols.
[0072] The VHT-STF field 250 is used to improve AGC estimation
performance in MIMO transmission.
[0073] The VHT-LTF field 260 is used for an STA to estimate a MIMO
channel. The next-generation WLAN system supports MU-MIMO
transmission. Thus, the VHT-LTF field 260 may be set by the number
of spatial streams where the PPDU 200 is transmitted. In addition,
if full channel sounding is supported and performed, the number of
VHT LTFs may be further increased.
[0074] The VHT-SIG B field 270 includes dedicated control
information necessary for a plurality of MIMO-paired STAs to
receive the PPDU 200 and obtain data. Accordingly, only when the
common control information included in the VHT-SIG B field 270
indicates that the PPDU 200 now received has been subjected to
MU-MIMO transmission, an STA may be designed to decode the VHT-SIG
B field 270. On the other hand, when the common control information
indicates that the PPDU 200 now received is for a single STA
(including SU-MIMO), an STA may be designed not to decode the
VHT-SIG B field 270.
[0075] The VHT-SIG B field 270 includes information about the
length of the PSDU included in the data field transmitted to each
STA, MCS information, and tail-related information included in the
data field. The VHT-SIG B field 270 further includes information
about encoding and rate-matching. The size of the VHT-SIG B field
270 may vary according to a type (MU-MIMO or SU-MIMO) of MIMO
transmission and a channel bandwidth used for PPDU
transmission.
[0076] The data field 280 includes data intended to be transmitted
to an STA. The data field 280 includes a PLCP Service Data Unit
through which an MAC Protocol Data Unit (MPDU) in the MAC layer has
been transmitted, a service field for initializing a scrambler, a
tail field including a bit sequence necessary to return a
convolution encoder to a zero state, and padding bits for
standardizing the length of the data field.
[0077] Meanwhile, in order to support a further improved
throughput, not only the downlink MU-MIMO transmission scheme, but
also an uplink MU-MIMO transmission scheme is to be supported. In a
WLAN system supporting the uplink MU-MIMO transmission scheme, a
plurality of STAs can transmit data to an AP at the same time. In
this case, the throughput of the WLAN system can be improved
because more traffic can be processed during not only downlink
communication, but also uplink communication.
[0078] A method of transmitting a data unit using the uplink
MU-MIMO transmission scheme is described below. The data unit means
a data field including a PSDU in a common PPDU format. Furthermore,
a preamble means a training field and a signal field for MIMO
transmission.
[0079] In the uplink MU-MIMO transmission scheme, a plurality of
STAs performs transmission and one AP performs reception. Thus, the
uplink MU-MIMO transmission scheme is relatively lower than the
downlink MU-MIMO transmission scheme in the degree of importance
related to the feedback of channel information. Form a viewpoint of
an AP, however, the AP needs to be accompanied by a method of
guaranteeing reception synchronization for radio signals
transmitted by respective STAs and a power control method for
preventing the deterioration of performance when there is a great
difference in the reception power of the radio signals.
Furthermore, there is a need for a method of reducing overhead
resulting from preambles transmitted by a plurality of STAs.
[0080] In order to support uplink MU-MIMO transmission in a WLAN
system, STAs that will transmit data units at the same time must be
specified. The STAs of the WLAN system may be operated in a PS
mode. The STAs are alternately operated in a doze state and an
awake state. Thus, if an STA of a doze state is selected as a
transmission STA, uplink traffic owned by the STA may not be
substantially processed.
[0081] In a WLAN system supporting uplink MU-MIMO transmission, a
method based on a contention based protocol may be taken into
account as a method of selecting STAs that will transmit data
units. A process of transmitting the data unit may include a
procedure in the random access period and a procedure in the data
transmission period. STAs having uplink traffic may transmit
Request To Send (RTS) frames to an AP in order to request data unit
transmission within the random access period, and only a
transmission STA group including an STA selected in a contention
with other STAs may transmit the data unit in the data transmission
period.
[0082] To this end, there is proposed a scheme in which an AP
designates a transmission STA group. Here, it is assumed that the
AP previously knows whether uplink traffic to be transmitted by
STAs associated with the AP exists. The AP may set several
transmission STA groups by grouping a plurality of STAs, and the
transmission STA group may be designated by a group ID. The group
ID for uplink MU-MIMO transmission is ID information which exists
separately from a group ID for downlink MU-MIMO transmission. The
two types of the group IDs may be identical with or different from
each other. An AP may change a list of member STAs, included in a
transmission STA group, according to a WLAN environment. An AP may
determine the number of spatial streams to be used by each of
member STAs, included in a transmission STA group indicated by a
group ID, and inform each STA of the number of spatial streams. The
term "group ID` hereinafter indicates a group ID for uplink which
indicates a transmission STA group for uplink MU-MIMO
transmission.
[0083] FIG. 3 is a flowchart illustrating a method of transmitting
a PPDU using an MU-MIMO transmission scheme according to an
embodiment of the present invention.
[0084] Referring to FIG. 3, it is assumed that an AP 310 has been
subjected to MU-MIMO paired with three STAs 321, 322, and 323. It
is assumed that all the STA1 321, the STA2 322, and the STA3 323
are STAs trying to perform uplink transmission.
[0085] The STA1 321 accesses a radio medium through a contention
and transmits an RTS frame to the AP 310 in order to request a data
unit to be transmitted (S311). The AP 310 transmits a Clear To Send
(CTS) frame to the STA1 321 in response to the RTS frame
(S312).
[0086] The STA1 321 receives the CTS frame and, after a Short
InterFrame Space (SIFS), requires information necessary to
determine whether the data unit will be transmitted according to a
single-user scheme or a multi-user scheme. Furthermore, the STA2
322 and the STA3 323 capable of receiving or overhearing an RTS
frame need to obtain information about whether uplink MU-MIMO
transmission has been scheduled. In general, it is the reason that
a STA having been received a CTS frame, even though the STA has not
transmitted a RTS frame, sets a Network Allocation Vector (NAV)
until data unit transmission is finished by the STA1 321 that
obtained a medium access right.
[0087] In order to provide the above information, there is proposed
a method of including information indicating whether an after data
transmission procedure is single user transmission or multi-user
transmission in the CTS frame transmitted by the AP 310. This
method is described with reference to FIG. 4.
[0088] FIG. 4 is a diagram showing a change of a scrambling
sequence for a CTS frame according to an embodiment of the present
invention.
CH_BANDWIDTH_IN_NON_HT and DYN_BANDWIDTH_IN_NON_HT of FIG. 4 are
the transmission/reception information parameters of an STA and an
AP. The value of the parameter CH_BANDWIDTH_IN_NON_HT may be CBW20,
CBW40, CBW80, or CBW160. The parameter CH_BANDWIDTH_IN_NON_HT is
used to modify the first 7 bits of the scrambling sequence to
indicate the duplicated bandwidth of a PPDU. The parameter
DYN_BANDWIDTH_IN_NON_HT is used to modify the first 7 bits of the
scrambling sequence when a transmission STA or a transmission AP or
both can operate a static or dynamic bandwidth.
[0089] Referring to FIG. 4, the first 7 bits of the scrambling
sequence are required in the PLCP DATA scrambling of the RTS/CTS
frames. The existing bits are configured as shown on the upper side
of FIG. 4. As shown on the lower side of FIG. 4, SU/MU indication
information of 1 bit may be included in the value of the first 7
bits of the scrambling sequence of the CTS frame in order to inform
reception STAs that which one of the single-user transmission
scheme and the multi-user transmission scheme will be used.
[0090] In addition, an STA trying to process uplink traffic may
inform an AP of information about whether the single user (SU)
transmission scheme will be used or the multi-user (MU)
transmission scheme will be used along with other STAs. Like in the
above method, this may be implemented by including SU/MU indicating
information in the value of the first 7 bits of the scrambling
sequence of the RTS frame as shown on the lower side of FIG. 4.
[0091] Meanwhile, in order to determine a transmission opportunity
(TXOP) period, an AP has to determine the TXOP period by taking a
TXOP limit value of traffic into account. The TXOP limit value is
dependent on an access category (AC) of traffic that will be
transmitted by an STA. There are two methods of the AP knowing the
TXOP limit value. One of the methods is that an AP obtains
information about the time when STAs can transmit packets through
the duration field of an RTS frame that has been transmitted by an
STA requesting transmission.
[0092] The other of the methods is that, since a TXOP limit value
is dependent on an AC of traffic, an STA informs an AP of the AC
that will be transmitted by the STA and the AP obtains information
about the TXOP limit value based in the AC. The first 7 bits of the
scrambling sequence are required in the PLCP DATA scrambling of an
RTS frame and a CTS frame. They are configured as shown on the
upper side of FIG. 4. Information about an AC may be included in
the value of the first 7 bits of the RTS/CTS scrambling sequence
and transmitted to an AP.
[0093] Referring back to FIG. 3, the AP 310 may determine a
transmission STA group including the STA1 321. The transmission STA
group may be specified by a group ID selected by the AP 310. The AP
310 determines the number of spatial streams to be allocated to
each of STAs included in the transmission STA group. The
information related to the group ID and the number of spatial
streams is control information necessary for uplink MU-MIMO
transmission, and the control information needs to be transmitted
to the transmission STA group. To this end, the AP 310 transmits a
VHT-SIG protection frame to the transmission STA group (S321, S322,
and S323). The VHT-SIG protection frame includes control
information necessary for uplink MU-MIMO transmission.
[0094] FIG. 5 is a block diagram showing the VHT-SIG protection
frame according to an embodiment of the present invention.
[0095] Referring to FIG. 5, the VHT-SIG protection frame 500
includes an L-STF 510, an L-LTF 520, an L-SIG field 530, a VHT-SIG
A1 field 540, and a VHT-SIG A2 field 550.
[0096] The L-STF field 510 is used for frame timing acquisition,
AGC convergence, coarse frequency acquisition, etc. The L-LTF field
520 is used to estimate a channel for de-modulating the L-SIG field
530, the VHT-SIG A1 field 540, and the VHT-SIG A2 field 550.
[0097] The L-SIG field 530 includes a rate subfield 531, a length
subfield 532, a parity subfield 533, and a tail subfield 534. The
rate subfield 531 indicates a data transmission rate which is
permitted for the AP 310 and the STAs. The length subfield 532
indicates a time interval in which the STAs can transmit data units
according to an uplink MU-MIMO transmission scheme. The parity
subfield 533 indicates a parity bit value, and the tail subfield
534 includes tail bits for returning an encoder to a zero
state.
[0098] The VHT-SIG A1 field 540 includes a bandwidth (BW) subfield
541, a Space Time Block Code (STBC) subfield 542, a group ID
subfield 543, the number-of-spatial streams (N.sub.SS) subfield 544
includes a tail subfield 545. The bandwidth subfield 541 indicates
a channel bandwidth to be used for uplink transmission. The STBC
subfield 542 indicates whether an STBC will be applied. The group
ID subfield 543 indicates a transmission STA group that will
transmit the data unit to the AP 310 according to the uplink
MU-MIMO transmission scheme. The number-of-spatial-stream subfield
544 indicates the number of spatial streams which has been
allocated to each of member STAs included in the transmission STA
group.
[0099] The VHT-SIG A2 field 550 includes information about whether
a short Guard Interval (GI) has been applied, information
indicating a coding method to be used by transmission STAs, an MCS
index, and information about beamforming.
[0100] STAs that have received the VHT-SIG protection frame 500 may
determine whether they are included in a transmission STA group
based on the group ID of the VHT-SIG A1 field 540. If an STA is
included in the transmission STA group, the STA can know the number
of spatial streams, allocated thereto, through the
number-of-spatial-streams subfield 544.
[0101] Referring back to FIG. 3, the STA1 321, the STA2 322, and
the STA3 323 correspond to member STAs included in a transmission
STA group selected by the AP 310. After receiving the VHT-SIG
protection frame, the STA1 321, the STA2 322, and the STA3 323
transmit data units to the AP 310 according to the uplink MU-MIMO
transmission scheme. Here, the states of uplink traffic owned by
the STAs may differ. Accordingly, uplink transmission procedures
performed by the STAs may be different.
[0102] FIG. 6 is a diagram showing a traffic queue for each of
STAs.
[0103] Referring to FIG. 6, it can be seen that the amount of
traffic accumulated in each of an STA1 321, an STA2 322, and an
STA3 323 is different. Each of the STAs calculates the amount of
traffic that may be transmitted through one aggregate MPDU (A-MPDU)
by using duration information received from an AP and divides the
traffic so that the amount of traffic is not exceeded. Here, since
only traffic belonging to an access category may be included in one
A-MPDU, traffic belonging to other AC should not be included in the
one A-MPDU.
[0104] As shown, assuming that each STA can transmit the amount of
traffic during three TXOP periods, from among pieces of traffic
included in the queue of each STA, the STAs process the traffic
through three PPDU transmission procedures during the TXOP
periods.
[0105] Each STA calculates an A-MPDU boundary by using a value
included in the duration field of the VHT-SIG protection frame. The
STA1 321 divides traffic related to AC_VI (video) according to the
A-MPDU boundary. The STA1 321 may process its own traffic through
three data unit transmissions. The STA2 322 has traffic related to
AC_VO (voice) and traffic related to AC_BE (best effort). The
traffic related to AC_VO must be divided into two and transmitted
because the traffic exceeds the A-MPDU boundary. The traffic
related to AC_BE may be transmitted through one data unit
transmission. Meanwhile, the traffic related to AC_BE may be
processed through a total of three data unit transmissions because
the traffic cannot be transmitted along with the traffic related to
AC_VO. The STA3 323 has AC_VI traffic, AC_VO traffic, and AC_BE
traffic. The STA3 323 may process the entire traffic through three
data unit transmissions because the amount of traffic for each AC
exists within the A-MPDU boundary.
[0106] Referring back to FIG. 3, each of the STA1 321, the STA2
322, and the STA3 323 divides relevant traffic according to the
A-MPDU boundary as shown in FIG. 6 and transmits a data unit,
including data related to the traffic, to the AP 310 along with a
preamble. The preamble is implemented by the VHT-STF of a downlink
MU-MIMO PPDU, a training sequence corresponding to a VHT-LTF, and a
VHT-SIG B field as shown in FIG. 2. The AP 310 may obtain timing
synchronization and improve AGC estimation through the VHT-STF and
the VHT-LTFs, obtain channel information, and then normally receive
the data unit. The VHT-SIG B field includes dedicated control
information according to an STA that must transmit the data unit,
as in the VHT-SIG B field of the PPDU format shown in FIG. 2. The
VHT-SIG B field may include a Modulation and Coding Scheme (MCS)
used by each STA in order to generate the data unit and information
about the length of the PSDU included in the data unit. The AP 310
may obtain data by demodulating and decoding a transmitted radio
signal by using the control information included in the VHT-SIG B
field.
[0107] As described above, a process of the AP 310 transmitting the
VHT-SIG protection frame to the STAs and, after a lapse of an SIFS,
the STAs transmitting the data units to the AP along with the
preambles becomes one PPDU transmission process. A combination of
the VHT-SIG protection frame and the format of the preamble and the
data unit transmitted by each STA is similar to the PPDU format
used for downlink MU-MIMO transmission in FIG. 2.
[0108] After a lapse of the SIFS since the VHT-SIG protection frame
transmitted by the AP 310 is received (S321), each of the STA1 321,
the STA2 322, and the STA3 323 transmits the preamble and the data
unit to the AP 310 (S331). The data unit transmitted by the STA1
321 includes data about traffic related to AC_VI. The data unit
transmitted by the STA1 322 includes data about traffic related to
AC_VO. The data unit transmitted by the STA1 323 includes data
about traffic related to AC_VI. After a lapse of an SIFS since the
preambles and the data units are received from the STAs, the AP
310, the AP 310 transmits an ACK message to the STA1 321, the STA2
322, and the STA3 323 (S341).
[0109] The AP 310 transmits the VHT-SIG protection frame to the
STA1 321, the STA2 322, and the STA3 323 again (S322). After a
lapse of an SIFS since the VHT-SIG protection frame is received,
each of the STA1 321, the STA2 322, and the STA3 323 transmits a
preamble and a data unit to the AP 310 (S332). The data unit
transmitted by the STA1 321 includes data about traffic related to
AC_VI. The data unit transmitted by the STA1 322 includes data
about traffic related to AC_VO. The data unit transmitted by the
STA1 323 includes data about traffic related to AC_VO. The AP 310
receives the preambles and the data units from the STAs and, after
a lapse of an SIFS, transmits an ACK message to the STA1 321, the
STA2 322, and the STA3 323 (S342).
[0110] The AP 310 transmits the VHT-SIG protection frame to the
STA1 321, the STA2 322, and the STA3 323 again (S323). The STA1
321, the STA2 322, and the STA3 323 receive the VHT-SIG protection
frame and, after a lapse of an SIFS, transmit respective preambles
and respective data units to the AP 310 (S333). The data unit
transmitted by the STA1 321 includes data about traffic related to
AC_VI. The data unit transmitted by the STA1 322 includes data
about traffic related to AC_BE. The data unit transmitted by the
STA1 322 includes data about traffic related to AC_BE. The AP 310
receives the preambles and the data units from the STAs and, after
a lapse of an SIFS, transmits an ACK message to the STA1 321, the
STA2 322, and the STA3 323 (S343).
[0111] The above three data transmission procedures are performed
within the obtained TXOP period. During each of the data
transmission intervals, each STA adds a preamble to a data unit
(i.e., a PSDU including data to be transmitted) and transmits the
data unit to an AP. Here, since the AP must be able to distinguish
the preambles, transmitted by the STAs, from each other, the STAs
must transmit the respective preambles using a method in which the
AP can distinguish the preambles from each other. As a method of
transmitting the preambles so that they can be distinguished from
each other, a Time Division Multiple Access (TDMA) scheme and a
Spatial Division Multiple Access (SDMA) scheme may be taken into
consideration.
[0112] FIG. 7 is a flowchart illustrating an example of a method of
transmitting a PPDU according to an embodiment of the present
invention.
[0113] Referring to FIG. 7, an AP 710 transmits a VHT-SIG
protection frame to an STA1 721, an STA2 722, and an STA3 723.
[0114] The STA1 721, the STA2 722, and the STA3 723 transmit only
their preambles according to a predetermined sequence. The
predetermined sequence may be determined according to the position
sequence of spatial streams allocated to a group ID. In this
example, after the STA1 721 transmits the preamble, the STA2 722
transmits the preamble and then the STA3 723 transmits the
preamble. After the STAs transmit all the preambles, they transmit
their PSDUs to the AP 710 at the same time.
[0115] A method based on the SDMA scheme may be used as the method
of transmitting a PPDU according to the embodiment of the present
invention. As a common solution when preambles overlap with each
other, orthogonal codes having a less cross correlation may be used
according to the sequence forming the STF/LTF of each STA. A set of
orthogonal codes is previously specified. When a transmission STA
group is determined by an AP, an STA may use a code according to
its sequence within the previously specified codes according to its
sequence.
[0116] Meanwhile, the lengths of the preambles transmitted by the
STAs may be different. A point of time at which the STAs transmit
the PSDUs is identical with a point of time at which the
transmission of the PSDU is finished. Accordingly, each of the STAs
may set a point of time at which the transmission of the PSDU is
finished by using a different method of transmitting the
preamble.
[0117] FIG. 8 is a flowchart illustrating an example of a method of
transmitting a PPDU based the SDMA scheme according to an
embodiment of the present invention. Referring to FIG. 8, each of
STAs 821, 822, and 823 differently sets a point of time at which a
preamble is transmitted to an AP 810. Meanwhile, a point of time at
which a PSDU is transmitted is identical with a point of time at
which the transmission of the PSDU is finished.
[0118] FIG. 9 is a flowchart illustrating another example of a
method of transmitting a PPDU based on SDMA according to an
embodiment of the present invention. Referring to FIG. 9, STAs 921,
922, and 923 start transmitting respective preambles to an AP 910
at the same time. Here, points of time at which the STAs 921, 922,
and 923 transmit respective PSDUs may become identical with each
other by adding dummy bits to VHT-LTF fields forming the respective
preambles. Accordingly, the transmission of the PSDUs by the STAs
can be finished at the same time.
[0119] FIG. 10 is a flowchart illustrating yet another example of a
method of transmitting a PPDU based on the SDMA scheme according to
an embodiment of the present invention. Referring to FIG. 10, STAs
1021, 1022, and 1023 start transmitting respective preambles to an
AP 1010 at the same time. In the example of FIG. 10, unlike in the
example of FIG. 9, the total lengths of VHT-LTFs transmitted by the
STAs 1021, 1022, and 1023 become identical with each other by
repeatedly adding a bit sequence forming a single VHT-LTF. Through
this process, a point of time at which each STA transmits a PSDU
may become identical with a point of time at which the transmission
of the PSDU is finished.
[0120] FIG. 11 is a block diagram showing a wireless apparatus in
which the embodiments of the present invention may be
implemented.
[0121] Referring to FIG. 11, the wireless apparatus 1100 includes a
processor 1110, memory 1120, and a transceiver 1130. The
transceiver 1130 transmits and receives a radio signal and
implements the physical layer of IEEE 802.11. The processor 1110 is
functionally coupled to the transceiver 1130 and is configured to
generate a data unit, including a preamble and a PSDU forming a
PPDU for uplink MU-MIMO transmission. The processor 1110 is set to
implement the MAC layer or the PHY layer or both which implements
the embodiments of the present invention shown in FIGS. 3 to
10.
[0122] The processor 1110 or the transceiver 1130 or both may
include Application-Specific Integrated Circuits (ASICs), other
chipsets, logic circuits and/or data processing units. When an
embodiment is implemented in software, the above scheme may be
implemented using a module (process, function, etc.) for performing
the above functions. The module may be stored in the memory 1120
and executed by the processor 1110. The memory 1120 may be included
in the processor 1110 or placed outside the processor 1110 and may
be functionally coupled to the processor 1110 by various known
means.
* * * * *